In general, the present invention is an integrated method for release and passivation of MEMS (micro-electro-mechanical systems) structures. In particular, the invention pertains to a method of improving the adhesion of a hydrophobic self-assembled monolayer (SAM) coating to a surface of a MEMS structure, for the purpose of preventing stiction.
Micromachining technology compatible with semiconductor processes is used to produce a number of devices such as piezoelectric motors containing cantilever beams, hinges, accelerometers, reflector antennae, microsensors, microactuators, and micromirrors, for example. One of the most popular microactuators is an electrostatic comb driver, due to its simplicity in fabrication and low power consumption. Surface micromachining fabrication processes for the electrostatic comb driver, as well as other beams and lever arms, have problems with stiction of such beams and lever arms to an underlying layer over which the beam or arm extends. The lever arm becomes deformed from its intended position, so that it does not extend out as desired. In the case of a membrane or diaphragm, the membrane or diaphragm becomes deformed from its intended position and may become stuck to an adjacent surface. Stiction is the number one yield-limiting problem in the production of the kinds of devices described above.
FIGS. 1A through 1C are simple schematics showing a cross-sectional side view of a starting structure for surface machining of a lever arm, the desired machined lever arm, and a lever arm which has been rendered non-functional due to stiction, respectively.
The FIG. 1A structure shows a substrate layer 102 (typically single crystal silicon), a portion of which is covered with a sacrificial layer 104 (typically silicon oxide), and a lever arm layer 106 (typically polysilicon) which is in contact with and adhered to substrate layer 102 at one end of lever arm layer 106. FIG. 1B shows the FIG. 1A structure after the removal of sacrificial layer 104 to produce the desired free-moving lever arm 107. The height xe2x80x9chxe2x80x9d of gap 108 between lever arm 107 and substrate 102, the length xe2x80x9clxe2x80x9d, and the cross-sectional thickness xe2x80x9ctxe2x80x9d of the lever arm 107 depend on the particular device in which the structure is employed. In many instances the relative nominal values of xe2x80x9chxe2x80x9d, xe2x80x9clxe2x80x9d, and xe2x80x9ctxe2x80x9d are such that capillary action during the fabrication process; or contaminants formed as byproducts of the fabrication process; or van der Waals forces; or electrostatic charges on the upper surface 110 of substrate layer 102 and/or on the undersurface 112 of lever arm layer 106, may cause lever arm 106 to become stuck to the upper surface 110 of substrate layer 102. This problem is referred to as xe2x80x9cstictionxe2x80x9d, and is illustrated in FIG. 1C. Stiction may occur during formation of the lever arm 107, or may occur subsequent to fabrication of the device and during packaging, shipment, or use (in-use stiction) of the device. A single crystal silicon or polysilicon surface of the kind which is frequently used to fabricate a lever arm, beam, membrane, or diaphragm is hydrophilic in nature, attracting moisture, which may cause stiction.
Stiction, which is the primary cause of low yield in the fabrication of MEMS devices, is believed to result from a number of sources, some of the most significant being capillary forces, surface contaminants, van der Waals forces, and electrostatic attraction. Factors which may contribute to stiction include: warpage due to residual stresses induced from materials; liquid-to-solid surface tension which induces collapse; drying conditions during processing; adverse and harsh forces from wet baths; aggressive designs (i.e., long and thin beams); surface-to surface attractions; inadequate cleaning techniques; aggressive cleaning techniques; and environments subsequent to fabrication, including packaging, handling, transportation, and device operation.
Various processes have been developed in an attempt to prevent stiction from occurring during fabrication of micromachined arms and beams. To reduce the possibility of stiction subsequent to release of a beam, lever arm, membrane, or diaphragm (so that it extends over open space), a surface treatment may be applied and/or a coating may be applied over freestanding and adjacent surfaces. For example, in U.S. Pat. No. 6,069,149, to Hetrick et al, issued Aug. 1, 2000, the inventors disclose a method for fabricating an adhesion-resistant microelectromechanical device. Amorphous hydrogenated carbon is used as a coating or structural material to prevent adhesive failures during the formation and operation of a microelectromechanical device. (Abstract) The amorphous hydrogenated carbon (AHC) coating is applied on the micromachined device after removal of the sacrificial layer and release of the structure. The sacrificial layer is removed in a wet etching solution such as hydrofluoric acid or buffered HF acid. (Col. 7, lines 26-32.) The method is said to reduce adhesive forces between microstructure surfaces by altering their surface properties. The AHC is said to create a hydrophobic surface, which results in lower capillary forces and an associated reduction in stiction. (Col. 2, lines 66-67, continuing at Col. 3, lines 1-4.)
U.S. Pat. No. 5,403,665, issued Apr. 4, 1995, to Alley et al., discloses a method of applying a self-assembled alkyltrichlorosilane monolayer lubricant to micromachines. Octadecyltrichlorosilane (OTS; C18H37SiCl3) is provided as an example of an alkyltrichlorosilane. In a dilute, non-polar, non-aqueous solution, OTS will deposit on silicon, polysilicon, and silicon nitride surfaces that have been previously treated to form a hydrophilic chemical oxide. Treatment of the silicon, polysilicon, or silicon nitride surfaces may be accomplished with an approximately 5 to 15 minute exposure to a hydrophilic chemical oxide promoter such as Piranha (H2O2:H2SO4), RCA SC-1, or room temperature H2O2. This treatment changes silicon and polysilicon surfaces from hydrophobic to hydrophilic. Thus, the surface will have a thin layer of adsorbed water. The OTS reacts with the thin adsorbed water layer that is present on the treated surface to form a single layer of molecules that are chemically bonded to the surface. (Col. 3, lines 23-40; Col. 4, lines 19-30)
The present invention pertains to the application of a hydrophobic, self-assembled monolayer (SAM) coating on a surface of a MEMS (micro-electro-mechanical systems) structure, for the purpose of preventing stiction. In particular, the invention pertains to a method of improving the adhesion of a hydrophobic SAM coating to a surface of a MEMS structure.
Self-assembled monolayer (SAM) coatings are known in the art. Self-assembly is a process in which a single, densely packed molecular layer of a material is selectively deposited on a fresh reactive surface. The process self-terminates after single layer coverage is achieved. SAM coatings are typically deposited from precursor long-chain hydrocarbons or fluorocarbons with a chlorosilane-based head, such as alkylchlorosilanes. Effective alkylchlorosilanes include OTS (octadecyltrichlorosilane; C18H37SiCl3), FDTS (perfluorodecyltrichlorosilane; C10H4F17SiCl3), and DMDS (dimethyldichlorosilane; (CH3)2SiCl2), by way of example, and not by way of limitation. The chemical structures of OTS and FDTS are shown in FIG. 2A (respectively indicated by reference numerals 200 and 210).
To improve the adhesion, prior to the application of a SAM coating, surfaces of a MEMS structure are treated with a plasma which was generated from a source gas comprising oxygen and, optionally, a source of hydrogen. The treatment oxidizes the surfaces, which are then reacted with hydrogen to form bonded OH groups on the surfaces. The hydrogen source may be present as part of the plasma source gas, so that the bonded OH groups are created during treatment of the surfaces with the plasma. Examples of hydrogen sources include NH3 or steam, by way of example and not by way of limitation. In the alternative, the plasma-treated, oxidized surfaces may be subsequently exposed to a gas containing a source of hydrogen, such as a mixture of hydrogen with an inert gas, or NH3, so that the oxidized surface reacts with the hydrogen to create bonded OH groups on the MEMS surfaces.
The plasma used to oxidize the MEMS structure surface should have a plasma density of about 1xc3x97108 exe2x88x92/cm3 or less at the substrate surface, and the plasma treatment should be carried out without a bias applied to the substrate. Typically, the plasma density is within the range of about 1xc3x97107 exe2x88x92/cm3 to about 1xc3x97108e xe2x88x92/cm3 at the substrate surface.
Typically, the plasma used to treat the MEMS structure surfaces is an externally generated plasma. The use of an external plasma generation source provides the ability to control the plasma to exhibit a low, yet uniform, ion density, preventing undesirable etching and/or ion bombardment of the MEMS structure surface during oxidation of the surface. The plasma pretreatment process of the invention is a very gentle, isotropic process which is performed for the sole purpose of preparing the surface for reaction with a SAM precursor. The surfaces may be silicon-containing surfaces or other surfaces within a MEMS structure, including, but not limited to, metal-containing surfaces. The highly isotropic process allows all exposed surfaces of the MEMS structure to be contacted with the plasma.
Oxygen typically makes up about 20 volume % to about 100 volume % of the reaction-generating portion of the plasma source gas. The source of hydrogen is typically NH3 or steam, by way of example, and not by way of limitation. If NH3 is used, the NH3 typically makes up about 0.1 volume % to about 20 volume % of the reaction-generating portion of the plasma source gas. More typically, the NH3 makes up about 0.5 volume % to about 10 volume % of the reaction-generating portion of the plasma source gas. The presence of nitrogen in the plasma source gas speeds up the rate of oxidation. Nitrogen (N2) may be present at about 20 volume % to about 80 volume % of the reaction-generating portion of the plasma source gas.
The plasma source gas may also include a nonreactive diluent gas, such as argon, helium, neon, xenon, krypton, and combinations thereof, for example, and not by way of limitation. The nonreactive diluent gas typically makes up about 20 volume % to about 80 volume % of the plasma source gas, with the remaining 80 volume % to 20 volume % being the reaction-generating portion of the plasma source gas.
FIG. 2A shows one example of a precursor 210 to a SAM coating, which is reacted with the surface 220 shown in FIG. 2B, to produce a SAM, as shown in FIG. 2C. FIG. 2B shows a hydrolyzed surface 220 of a MEMS structure. During application of a SAM coating, the chlorosilane-based head of an alkylchlorosilane, shown as 212 in FIG. 2A, may be reacted with the hydrolyzed surface, shown as 220 in FIG. 2B, liberating one molecule of HCl for each Sixe2x80x94Cl bond that is hydrolyzed. FIG. 2C shows a MEMS surface 230 on which a self-assembled monolayer of individual FDTS molecules 210 has been grown. A similar structure may be achieved for a self-assembled monolayer of individual OTS molecules.
Also disclosed herein is an integrated method for release and passivation of a MEMS structure which includes the pretreatment process prior to SAM application which was described above. According to the integrated process, a substrate including at least one MEMS structure is loaded into a processing chamber. A first pretreatment step includes contacting the substrate with a plasma which is generated from a source gas comprising oxygen. This pretreatment removes any moisture, particles, or contaminants present on the substrate surface prior to MEMS release. A release process is then performed, during which a sacrificial layer present within the MEMS structure is removed. The release process is typically a cyclic etch/cleaning procedure, where release is accomplished by plasmaless etching of a sacrificial layer material, followed by a cleaning step in which byproducts from the etch process and other contaminants which may lead to stiction are removed. A second pretreatment, comprising contacting surfaces of the MEMS structure with a plasma generated from a source gas comprising oxygen and, optionally, a source of hydrogen. The treatment oxidizes the surfaces, which are then reacted with hydrogen to form bonded OH groups on the surfaces. The hydrogen source may be present as part of the plasma source gas, so that the bonded OH groups are created during treatment of the surfaces with the plasma. Examples of hydrogen sources include NH3 or steam, by way of example and not by way of limitation. In the alternative, the plasma-treated, oxidized surfaces may be subsequently exposed to a gas containing a source of hydrogen, such as a mixture of hydrogen with an inert gas, or NH3, so that the oxidized surface reacts with the hydrogen to create bonded OH groups on the MEMS surfaces.
Subsequent to formation of the hydroxyl groups, the MEMS structure surfaces are exposed to a reactant which produces a self-assembled monolayer (SAM) coating. During application of the SAM coating to surfaces of the MEMS structure, a SAM coating typically forms on surfaces of the processing chamber. This SAM coating needs to be removed from surfaces of the processing chamber prior to the performance of subsequent processing steps within the chamber. Therefore, after removal of the substrate from the chamber, a chamber cleaning step is typically carried out, in which surfaces of the processing chamber are contacted with a plasma generated from a source gas comprising oxygen, whereby residual SAM is removed from processing chamber surfaces.